Welcome to the Q&A Forum

Who owns the URL where the blog is located? If they own it then they are being paid by you to build a brand that competes against you... and when you stop paying they will own a share of your business SERPs. If you own it then you simply redirect the content to a folder on your own site and tell them to continue promoting. Going back to the situation of them being the owner....  I think that you need to let them know that the content and promotion that you are paying for will be immediately redirected to a folder on your primary site.

Many thanks Ryan.